Renown got her range increase primarily from the 1935 major rebuild which included an engineering upgrade - similar to what was done to the QE, Valiant, and Warspite. The Renown was originally built with 42x boilers of the 1915 era technology/efficiency. During the 1935 rebuild the 42x old boilers were replaced with 8x 1935 technology/efficiency boilers. Her rated SHP also increased, from 112,000 to 120,000. Range as built in WWI was around 4000 nm, and about 6600 nm afterward, as mentioned above.
The Repulse never got the upgrade, and her engineering plant was worn out by the start of WWII, hence the reduced range of 3600 vs the original 4000 nm.